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Polymerization of the E and Z Isomers of Bis-(Triethoxysilyl)-2-Butene

Description: We have synthesized the Z and E isomers of 1,4-bis(triethoxysilyl)-2- butene and polymerized them under acid and base catalyzed sol-gel conditions. As expected the E system formed crosslinked, insoluble gels. The Z isomer, by nature of its geometry, formed high molecular weight, soluble polymeric products under acidic conditions. We were able to prepare and isolate both the cyclic disilsesquioxane monomer, and its dimer. Comparison of their spectral characterization with that of the soluble polymers suggests that the cyclics are present within the polymers. lle synthesis of a dimer likely present at some early stage of the polymerization suggests that we may be able to control the reaction and form rigid polymers with controllable tacticity. In addition, most of the gels were found to be non-porous indicating that the gels were, in fact, more compliant than ethenylene-bridged polysilsesquioxanes leading to collapse of pores during drying.
Date: May 11, 1999
Creator: Carpenter, J.P. Dorhout, K.; Loy, D.A.; Shaltout, R.M. & Shea, K.J.
Partner: UNT Libraries Government Documents Department

Experimental and Computational Studies of the Isomerization Reactions of Bidentate Phosphine Ligands in Triosmium Clusters: Kinetics of the Rearrangements from Bridged to Chelated Isomers and X-ray Structures of the Clusters Os3 (CO)10 (dppbz), 1,1-Os3 (CO)10 (dppbzF4), HOs3 (CO)9 [μ -1,2-PhP (C6H4-ɳ1) C6H4PPh2], and HOs3 (CO)9- [μ-1,2-PhP (C6H4-ɳ 1) C6F4PPh2]

Description: Article on experimental and computational studies of the isomerization reactions of bidentate phosphine ligands in triosmium clusters.
Date: February 22, 2011
Creator: Zhang, Xue; Kandala, Srikanth; Yang, Li; Watson, William H.; Wang, Xiaoping; Hrovat, David A. et al.
Partner: UNT College of Arts and Sciences

Simultaneously Bound Guests and Chiral Recognition: A Chiral Self-Assembled Supramolecular Host Encapsulates Hydrophobic Guests

Description: Driven by the hydrophobic effect, a water-soluble, chiral, self-assembled supramolecular host is able to encapsulate hydrophobic organic guests in aqueous solution. Small aromatics can be encapsulated in the supramolecular assembly, and the simultaneous encapsulation of multiple guests is observed in many cases. The molecular host assembly is able to recognize different substitutional isomers of disubstituted benzenes with ortho substitution leading to the encapsulation of two guests, but meta or para substitution leading to the encapsulation of only one guest. The scope of hydrophobic guest encapsulation is further explored with chiral natural product guests. Upon encapsulation of chiral guests into the racemic host, diastereomeric host-guest complexes are formed with observed diastereoselectivities of up to 78:22 in the case of fenchone.
Date: March 6, 2008
Creator: Hastings, Courtney J.; Pluth, Michael D.; Biros, Shannon M.; Bergman, Robert G. & Raymond, Kenneth N.
Partner: UNT Libraries Government Documents Department

Isomer Energy Differences for the C4H3 and C4H5 Isomers UsingDiffusion Monte Carlo

Description: A new diffusion Monte Carlo study is performed on the isomers of C{sub 4}H{sub 3} and C{sub 4}H{sub 5} emulating the methodology of a previous study [Int. J. Chem. Kinetics 33, 808 (2001)]. Using the same trial wave function form of the previous study, substantially different isomerization energies were found owing to the use of larger walker populations in the present work. The energy differences between the E and I isomers of C{sub 4}H{sub 3} were found to be 10.5 {+-} 0.5 kcal/mol and for C{sub 4}H{sub 5}, 9.7 {+-} 0.6 kcal/mol. These results are in reasonable accord with recent MRCI and CCSD(T) findings.
Date: December 1, 2007
Creator: Domin, D.; Lester Jr., W.A.; Whitesides, R. & Frenklach, M.
Partner: UNT Libraries Government Documents Department

Coexistence of a weakly-deformed band in a strongly-deformednucleus

Description: A weakly-deformed band J{sup {pi}} (E{sub x} keV) 0+ (1182), 2+ (1418), 4+ (1701) is identified in the strongly-deformed nucleus, {sup 154}Gd. Detailed {gamma}-ray spectroscopy following the beta decays of {sup 154}Eu (J = 3), {sup 154g,m1,m2}Tb (J = 0, 3, 7) are used to establish this structure. The structure is explained in terms of a pairing isomer which results from the {nu}[505] {up_arrow} Nilsson intruder orbital.
Date: October 27, 2002
Creator: Kulp, W.D.; Wood, J.L.; Krane, K.S.; Loats, J.; Schmelzenbach,P.; Stapels, C.J. et al.
Partner: UNT Libraries Government Documents Department

Enantioselective Intramolecular Hydroarylation of Alkenes via Directed C-H Bond Activation

Description: Highly enantioselective catalytic intramolecular ortho-alkylation of aromatic imines containing alkenyl groups tethered at the meta position relative to the imine directing group has been achieved using [RhCl(coe){sub 2}]{sub 2} and chiral phosphoramidite ligands. Cyclization of substrates containing 1,1- and 1,2-disubstituted as well as trisubstituted alkenes were achieved with enantioselectivities >90% ee for each substrate class. Cyclization of substrates with Z-alkene isomers proceeded much more efficiently than substrates with E-alkene isomers. This further enabled the highly stereoselective intramolecular alkylation of certain substrates containing Z/E-alkene mixtures via a Rh-catalyzed alkene isomerization with preferential cyclization of the Z-isomer.
Date: May 22, 2008
Creator: Harada, Hitoshi; Thalji, Reema; Bergman, Robert & Ellman, Jonathan
Partner: UNT Libraries Government Documents Department

Atomic-Nuclear Coupling Experiments

Description: Atomic-nuclear coupling experiments are described, with an emphasis on recent experiments aimed at demonstrating the NEET mechanism in atomic nuclei. Upper limits for x-ray induced decay of the Hf-178 31-y isomer reported by Ahmad and his colleagues are presented, and these upper limits are contrasted with the positive reports of Collins and coworkers.
Date: October 25, 2005
Creator: Becker, J A
Partner: UNT Libraries Government Documents Department

ISOMERIC-YIELD RATIOS IN NUCLEAR REACTIONS. (WITH A COMPILATION OF EXPERIMENTAL DATA)

Description: Experimertal procedures are briefly described for the determination of the ratios of cross sections for the formation of isomeric states in nuclear reactions. Much avatlable experimertal data on isomeric yield ratios in nuclear reactions is compiled. Some of the collected data are discussed in terms of angular-momentum effects in nuclear reactions. (230 references).
Date: September 1, 1962
Creator: Wing, J.
Partner: UNT Libraries Government Documents Department

Near Degenerate Rearrangement Between the Radical Cations of Formaldehyde and Hydroxymethylene

Description: Motivated by the recent experiments of Berkowitz, a systematic theoretical study of the ion isomerization H{sub 2}CO{sup +} {yields} HCOH{sup +} has been carried out. Structures and vibrational frequencies for H{sub 2}CO{sup +}, the transition state, and the cis and trans isomers of HCOH{sup +} have been determined at the double zeta basis set self-consistent-field (SCF) level of theory. Equilibrium geometries were also predicted from SCF theory using a double zeta plus polarization (DZ+P) basis set. Final energetics were pinned down using DZ+P configuration interaction, involving a total of 16,290 configurations. The most reliable theoretical results suggest that trans-HCOH{sup +} lies 5.5 kcal above H{sub 2}CO{sup +}. Zero-point vibrational energy corrections do not change this H{sub 2}CO{sup +}-HCOH{sup +} separation. Similarly cis-HCOH{sup +} is predicted to lie 4.1 kcal above the trans isomer, and the barrier to rotation between the two HCOH{sup +} isomers is -18 kcal. The barrier to H{sub 2}CO{sup +} {yields} HCOH{sup +} rearrangement is predicted to be 49.0 kcal, or 44.4 kcal after correction for zero-point vibrational energies is made. The relationship between this cationic 1,2-hydrogen shift and the corresponding neutral rearrangement is discussed in terms of qualitative molecular orbital theory.
Date: June 1, 1980
Creator: Osamura, Y.; Goddard, J.D.; Schaefer III, H.F. & Kim, K.S.
Partner: UNT Libraries Government Documents Department

On the Ionization Energies of C4H3 Isomers

Description: We have conducted a combined experimental and theoretical study on the formation of distinct isomers of resonantly stabilized free radicals, C4H3, which are important intermediates in the formation of polycyclic aromatic hydrocarbons in combustion flames and possibly in the interstellar medium. Our study utilized laser ablation of graphite in combination with seeding the ablated species in neat methylacetylene gas which also acted as a reagent. Photoionization efficiency (PIE) curves were recorded of the C4H3 isomers at the Advanced Light Source from 8.0 to 10.3 eV. The experimental PIE curve was compared with theoretical ones suggesting the formation of four C4H3 radicals: two acyclic structures i-C4H3 [1] and E/Z-n-C4H3 [2E/2Z]and two cyclic isomers 3 and 4. These molecules are likely formed via an initial addition of ground state carbon atoms to the carbon-carbon triple bond of the methylacetylene molecule followed by isomerization via hydrogen migrations and ring opening and emission of atomic hydrogen from these intermediates.
Date: September 16, 2009
Creator: Kaiser, Ralf I.; Mebel, Alexander; Kostko, Oleg & Ahmed, Musahid
Partner: UNT Libraries Government Documents Department

Improved resolution of hydrocarbon structures and constitutional isomers in complex mixtures using Gas Chromatography-Vacuum Ultraviolet-Mass Spectrometry (GC-VUV-MS)

Description: Understanding the composition of complex hydrocarbon mixtures is important for environmental studies in a variety of fields, but many prevalent compounds cannot be confidently identified using traditional gas chromatography-mass spectrometry (GC-MS) techniques. This work uses vacuum-ultraviolet (VUV) ionization to elucidate the structures of a traditionally"unresolved complex mixture" by separating components by GC retention time, tR, and mass-to-charge ratio, m/Q, which are used to determine carbon number, NC, and the number of rings and double bonds, NDBE. Constitutional isomers are resolved based on tR, enabling the most complete quantitative analysis to date of structural isomers in an environmentally-relevant hydrocarbon mixture. Unknown compounds are classified in this work by carbon number, degree of saturation, presence of rings, and degree of branching, providing structural constraints. The capabilities of this analysis are explored using diesel fuel, in which constitutional isomer distribution patterns are shown to be reproducible between carbon numbers and follow predictable rules. Nearly half of the aliphatic hydrocarbon mass is shown to be branched, suggesting branching is more important in diesel fuel than previously shown. The classification of unknown hydrocarbons and the resolution of constitutional isomers significantly improves resolution capabilities for any complex hydrocarbon mixture.
Date: September 13, 2011
Creator: Isaacman, Gabriel; Wilson, Kevin R.; Chan, Arthur W. H.; Worton, David R.; Kimmel, Joel R.; Nah, Theodora et al.
Partner: UNT Libraries Government Documents Department

Improved resolution of hydrocarbon structures and constitutional isomers in complex mixtures using Gas Chromatography-Vacuum Ultraviolet-Mass Spectrometry (GC-VUV-MS) (Supplementary Info)

Description: Understanding the composition of complex hydrocarbon mixtures is important for environmental studies in a variety of fields, but many prevalent compounds cannot be confidently identified using traditional gas chromatography-mass spectrometry (GC-MS) techniques. This work uses vacuum-ultraviolet (VUV) ionization to elucidate the structures of a traditionally"unresolved complex mixture" by separating components by GC retention time, tR, and mass-to-charge ratio, m/Q, which are used to determine carbon number, NC, and the number of rings and double bonds, NDBE. Constitutional isomers are resolved based on tR, enabling the most complete quantitative analysis to date of structural isomers in an environmentally-relevant hydrocarbon mixture. Unknown compounds are classified in this work by carbon number, degree of saturation, presence of rings, and degree of branching, providing structural constraints. The capabilities of this analysis are explored using diesel fuel, in which constitutional isomer distribution patterns are shown to be reproducible between carbon numbers and follow predictable rules. Nearly half of the aliphatic hydrocarbon mass is shown to be branched, suggesting branching is more important in diesel fuel than previously shown. The classification of unknown hydrocarbons and the resolution of constitutional isomers significantly improves resolution capabilities for any complex hydrocarbon mixture.
Date: September 5, 2011
Creator: Aerosol Dynamics Inc.,; Aerodyne Research, Inc.,; Tofwerk AG, Thun, Switzerland; Isaacman, Gabriel; Wilson, Kevin R.; Chan, Arthur W. H. et al.
Partner: UNT Libraries Government Documents Department

Direct observation of photoinduced bent nitrosyl excited-state complexes

Description: Ground state structures with side-on nitrosyl ({eta}{sup 2}-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited state structures with bent-NO ligands have been proposed for years but never directly observed. Here we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO){sub 3}(NO), a model transition-metal-NO compound. Surprisingly, we have observed no evidence for ON and {eta}{sup 2}-NO structural isomers, but have observed two bent-NO complexes. DFT modeling of the ground and excited state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO){sub 3}(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure.
Date: June 28, 2008
Creator: Sawyer, Karma R.; Steele, Ryan P.; Glascoe, Elizabeth A.; Cahoon, James F.; Schlegel, Jacob P.; Head-Gordon, Martin et al.
Partner: UNT Libraries Government Documents Department

The mechanism for iron-catalyzed alkene isomerization in solution

Description: Here we report nano- through microsecond time-resolved IR experiments of iron-catalyzed alkene isomerization in room-temperature solution. We have monitored the photochemistry of a model system, Fe(CO){sub 4}({eta}{sup 2}-1-hexene), in neat 1-hexene solution. UV-photolysis of the starting material leads to the dissociation of a single CO to form Fe(CO){sub 3}({eta}{sup 2}-1-hexene), in a singlet spin state. This CO loss complex shows a dramatic selectivity to form an allyl hydride, HFe(CO){sub 3}({eta}{sup 3}-C{sub 6}H{sub 11}), via an internal C-H bond-cleavage reaction in 5-25 ns. We find no evidence for the coordination of an alkene molecule from the bath to the CO loss complex, but do observe coordination to the allyl hydride, indicating that it is the key intermediate in the isomerization mechanism. Coordination of the alkene ligand to the allyl hydride leads to the formation of the bis-alkene isomers, Fe(CO){sub 3}({eta}{sup 2}-1-hexene)({eta}{sup 2}-2-hexene) and Fe(CO){sub 3}({eta}{sup 2}-1-hexene){sub 2}. Because of the thermodynamic stability of Fe(CO){sub 3}({eta}{sup 2}-1-hexene)({eta}{sup 2}-2-hexene) over Fe(CO){sub 3}({eta}{sup 2}-1-hexene){sub 2} (ca. 12 kcal/mol), nearly 100% of the alkene population will be 2-alkene. The results presented herein provide the first direct evidence for this mechanism in solution and suggest modifications to the currently accepted mechanism.
Date: May 27, 2008
Creator: Sawyer, Karma R.; Glascoe, Elizabeth A.; Cahoon, James F.; Schlegel, Jacob P. & Harris, Charles B.
Partner: UNT Libraries Government Documents Department

Cyanide Effects on Carbon Dioxide Fixation in Chlorella

Description: Green algae have been treated with radioactive KCN in an investigation of the effect of cyanide on photosynthesis. A multitude of products have been found to be formed in very short exposures (10 to 15 sec). One of these products has been identified with the product formed when the algae are given radioactive CO{sub 2} and nonradioactive KCN. The same product has been synthesized by a nonenzymatic cyanohydrin addition reaction on ribulose-1, 5-diphosphate. It has been shown to be a 2-carboxy-pentitol (probably mostly ribitol)-1, 5-diphosphate. Upon hydrolysis it gives an hydroxy acid (or mixture of isomers) closely related to hamamelonic acid. The significance of this and the other as yet unidentified products of cyanide interaction with a biological system is discussed with respect to the use of cyanide as an inhibitor.
Date: December 17, 1957
Creator: Rabin, Bernard R.; Shaw, D. F.; Pon, Ning G.; Anderson, J. M. & Calvin, M.
Partner: UNT Libraries Government Documents Department

Reinvestigation of the Direct Two-proton Decay of the Long-lived Isomer 94Agm [0.4 s, 6.7 MeV, (21+)]

Description: An attempt to confirm the reported direct one-proton and two-proton decays of the (21+) isomer at 6.7(5) MeV in 94Ag has been made. The 0.39(4) s half-life of the isomer permitted use of a helium-jet system to transport reaction products from the 40Ca + natNi reaction at 197 MeV to a low-background area; 24 gas Delta E-(Si) E detector telescopes were used to identify emitted protons down to 0.4 MeV. No evidence was obtained for two-proton radioactivity with a summed energy of 1.9(1) MeV and a branching ratio of 0.5(3)percent. Two groups of one-proton radioactivity from this isomer had also been reported; our data confirm the lower energy group at 0.79(3) MeV with its branching ratio of 1.9(5)percent.
Date: March 5, 2009
Creator: Cerny, J.; Moltz, D. M.; Lee, D. W.; Perajarvi, K.; Barquest, B. R.; Grossman, L. E. et al.
Partner: UNT Libraries Government Documents Department